Gaseous hydrides such as arsine, phosphine, hydrogen selenide, silane, hydrogen sulfide and diborane are important as a raw material for producing a compound semiconductor such as gallium-arsenic (GaAs) or gallium-phosphorus (GaP) and an ion implantation gas, and the mount thereof used is increasing year by year. At the same time, with the increase of the integration extent of a semiconductor device, gaseous hydrides having an extremely low content of impurities have been required.
Gaseous hydrides for use in the production of semiconductors are generally commercially available as the pure gaseous hydride or in the form of being diluted with hydrogen gas or an inert gas.
These gaseous hydrides contain oxygen and moisture as impurities and of these impurities, moisture can be removed by a dehumidifying agent such as a synthetic zeolite.
The oxygen content in commercially available gaseous hydrides is usually 10 ppm or less, but, recently, gaseous hydrides contained in a bomb having a relatively low oxygen content of from 0.1 to 0.5 ppm are commercially available.
There are almost no methods for efficiently removing oxygen contained in gaseous hydrides but a method for purifying arsine by bringing a material having an adsorptive power to arsine, such as active carbon or a synthetic zeolite, into contact with arsine to reduce the oxygen content to 1 ppm or less is proposed in, for example, JP-A-62-78116 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
However, gaseous hydrides having an oxygen content to an extent of slightly below 1 ppm cannot sufficiently meet the requirement in a recent semiconductor production process and it has strongly been desired to reduce the oxygen content in gaseous hydrides to 0.1 ppm or less.
Further, since such gaseous hydrides are sometimes contaminated by the entrance of impurities such as air in supplying steps to an apparatus for producing semiconductors such as a step of connecting a gaseous hydride bomb to the apparatus or a step of changing pipes, it is desired to finally remove impurities immediately before the apparatus.
On the other hand, a method for removing oxygen contained in a crude gaseous hydride has been proposed, which comprises bringing the crude gaseous hydride into contact with at least one of nickel arsenides, nickel phosphides, nickel silicides, nickel selenides and nickel borides, as described in U.S. patent application Ser. No. 07/421,750 filed Sept. 26, 1989. This method can efficiently remove oxygen to a concentration of 0.01 ppm or less.
However, it has been found that in the case of using the above-described nickel compounds, part of the gaseous hydrides is captured by those compounds at the initial stage of gas flowing, and the gas concentration at outlet lowers at the initial stage of purification, depending on the conditions.
Therefore, if the amount of flow of the gas is large, the gas concentration at outlet reaches the desired concentration within a relatively short period of time and thus, there is no particular problem. However, if the amount of flow of the gas is small, e.g., about 1 cm/sec in terms of a linear velocity (LV), there is the problem that a substitution of the gas is required for a long period of time until initiating to feed the gas to a process.
Further, in the semiconductor production step, when a one-day work is completed, the feed lines of gas are generally replaced by an inert gas such as nitrogen gas and left as it is to the next day, and the type of the gas is often changed during the operation of apparatus.
Therefore, it is demanded by semiconductor manufactures that impurities contained in the gaseous hydrides should be efficiently removed, and at the same time, the time required to reach the gas concentration to the desired concentration should be within at least 1 hour and preferably within 30 minutes from the initiation of flowing the gas.